tro, chemistry: a molecular approach1 supercritical fluid as a liquid is heated in a sealed...

Tro, Chemistry: A Molecular Approach 1

Supercritical Fluid• as a liquid is heated in a sealed container, more vapor collects

causing the pressure inside the container to rise and the density of the vapor to increase and the density of the liquid to decrease

• at some temperature, the meniscus between the liquid and vapor disappears and the states commingle to form a supercritical fluid

• supercritical fluid have properties of both gas and liquid states

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http://www.youtube.com/watch?v=yBRdBrnIlTQ

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The Critical Point• the temperature required to produce a

supercritical fluid is called the critical temperature

• the pressure at the critical temperature is called the critical pressure

• at the critical temperature or higher temperatures, the gas cannot be condensed to a liquid, no matter how high the pressure gets

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Sublimation and Deposition• molecules in the solid have thermal energy that allows

them to vibrate• surface molecules with sufficient energy may break

free from the surface and become a gas – this process is called sublimation

• the capturing of vapor molecules into a solid is called deposition

• the solid and vapor phases exist in dynamic equilibrium in a closed containerat temperatures below the melting point therefore, molecular solids have a vapor pressure

solid gassublimation

deposition

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Sublimation

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Melting = Fusion

• as a solid is heated, its temperature rises and the molecules vibrate more vigorously

• once the temperature reaches the melting point, the molecules have sufficient energy to overcome some of the attractions that hold them in position and the solid melts (or fuses)

• the opposite of melting is freezing

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Heating Curve of Water

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Phase Diagrams• describe the different states and state changes that

occur at various temperature - pressure conditions• areas represent states• lines represent state changes

liquid/gas line is vapor pressure curveboth states exist simultaneouslycritical point is the furthest point on the vapor pressure

curve• triple point is the temperature/pressure condition

where all three states exist simultaneously• for most substances, freezing point increases as

pressure increases

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Phase DiagramsP

ress

ure

Temperature

vaporization

condensation

criticalpoint

triplepoint

Solid Liquid

Gas

1 atm

normalmelting pt.

normalboiling pt.

Fusion Curve

Vapor PressureCurve

SublimationCurve

melting

freezing

sublimation

deposition

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Phase Diagram of Water

Temperature

Pre

ssur

e

criticalpoint

374.1°C217.7 atm

triplepoint

Ice Water

Steam

1 atm

normalboiling pt.

100°C

normalmelting pt.

0°C

0.01°C0.006 atm

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Water – An Extraordinary Substance• water is a liquid at room temperature

most molecular substances with small molar masses are gases at room temperature

due to H-bonding between molecules

• water is an excellent solvent – dissolving many ionic and polar molecular substances because of its large dipole moment even many small nonpolar molecules have solubility in water

e.g., O2, CO2

• water has a very high specific heat for a molecular substance moderating effect on coastal climates

• water expands when it freezes at a pressure of 1 atm

about 9% making ice less dense than liquid water